Understanding Inter-AS MPLS VPN Option C (Solution 2)

In MPLS L3VPN Inter-AS Option C, the Route Reflectors (RRs) are used for VPN route distribution but do not participate in data forwarding. The traffic forwarding path is based on MPLS labels assigned at different routers.

Each data packet carries two MPLS labels:

Transport Label (T1) -- Used for forwarding within the backbone.

VPN Label (V1) -- Used for VPN route identification.

Breaking Down the Labels:

VPN Label (V1) -- Allocated by PE1 (Answer: C )

V1 (Inner Label) is the VPN label assigned by the egress PE router (PE1).

This label ensures that when the packet reaches PE1, it knows how to forward the traffic to CE1 (192.168.1.1).

Transport Label (T1) -- Allocated by P2 (Answer: B )

T1 (Outer Label) is the transport label assigned by P2 to forward the packet across the AS core.

This label ensures the packet reaches the next-hop router (ASBR-PE1 in AS100).

Why Are the Other Options Incorrect?

A. 'T1 is allocated by RR2' (Incorrect)

RR2 is used only for route reflection; it does not participate in label allocation or forwarding.

D. 'V1 is allocated by P2' (Incorrect)

P2 is an intermediate router and does not allocate VPN labels.

VPN labels are always assigned by the egress PE router (PE1 in AS100).

Reference: Huawei HCIE Datacom -- MPLS L3VPN Inter-AS Option C